Ab-initio study of the beryllium-sulphur and beryllium-nitrogen co-doped graphene for nanoelectronic and optoelectronic devices

Abstract

Silicon and indium tin oxide (ITO) are the active components of the modern day devices. ITO is the most used transparent conducting material (TC) in smartphones and other touch panel devices, because the required properties of TCs such as low sheet resistance, high optical transparency, and stability found in ITO are difficult to match by other materials. However, due to its limited geographical availability, susceptibility to conductivity degradation, rising price and limited flexibility, which does not favour the demand for flexible devices, there is a need for an ideal replacement for ITO and likewise for silicon. Silicon has been the base material in microelectronics for over 49 years. However, as a result of the rising demand for miniaturized flexible devices further scaling of silicon for use in the active developing field of nanoelectronics might lead to performance restriction due to overheating and current leakage through the gate. Graphene has a stable structure, high charge carrier mobility, good thermal conductivity, high optical transparency, and high tensile strength of 130.5 GPa. In fact, it is the strongest material ever to be tested. Due to these fascinating properties, graphene has been proposed as a potential replacement for silicon and ITO for use in nanoelectronic and optoelectronic devices. However, despite these outstanding properties, it has no band-gap which makes it unsuitable for a direct integration in nanoelectronic devices. Aside from these limitations, graphene also has high sheet resistance and lower conductivity compared to ITO. These drawbacks likewise limit its application as a TC. Substitutional doping of graphene with heteroatoms has been extensively reported as a facile approach for tailoring the properties in order to increase its applicability range to the field of nanoelectronics and optoelectronics. Despite the gigantic stride that has been achieved through first-principles calculations in predicting nanomaterials that satisfy the aforementioned applications, synthesizing experimentally such heteroatom-doped graphene with the required specifications remains a contending issue. As a result, other heteroatom-doped graphene are being explored to determine if they would be amenable for synthesis experimentally. In this study, for the first time, ab initio calculations within the framework of density functional theory were performed to study the vibrational, electronic structure, structural and optical properties of beryllium/nitrogen (Be-N) and beryllium/sulphur (Be-S) co-doped graphene. It is observed that Be-S co-doped graphene is thermodynamically stable, has no metallic character and the band-gap can be tuned from zero to 0.7 eV by increasing the impurity concentration. A minimum band-gap of 0.4 eV is required for ON/OFF ratio in a transistor with graphene platform. Thus, the calculated value of the band-gap of Be-S co-doped graphene meets this specification. In addition, Be-N co-doped graphene was found to be also thermodynamically stable due to the absence of negative frequencies in the phonon dispersion. Interestingly, it exhibits both metallic and semiconducting character, and the band-gap can be tuned from zero up to 1.88 eV depending on the impurity concentration of the system. The presence of metallic character implies that the system is highly conductive as compared to pristine graphene. Moreover, the analysis of the optical spectrum shows that the system is transparent within the optical frequency of 7.0-10 eV for the parallel polarisation of the electromagnetic field irrespective of the impurity concentration. Thus, the interesting properties of Be-N co-doped graphene make it an alternative proposition as a replacement for ITO. However further research is needed to determine the work-function of this material to know if the application as a transparent electrode material in a photovoltaic is imminent. This study contributes to the on-going research of finding alternative nanomaterials to replace silicon and ITO for use in the field of nanoelectronics and optoelectronics respectively.